The present invention relates generally to systems for diagnosing engine thermostat in a vehicle equipped with an internal combustion engine.
Vehicle cooling systems typically have a coolant temperature sensor for providing coolant temperature information to the electronic engine controller and a thermostat for providing constant coolant temperature control. The purpose of the cooling system is to remove surplus heat from the engine, maintain an even and efficient heat level, and to bring the cold engine up to the efficient heat level as soon as possible after starting. When the engine is cold, the thermostat closes and confines the coolant to the engine, enabling it to heat up quickly. When the coolant reaches a predetermined temperature, the thermostat starts to open and allows coolant to circulate. The thermostat constantly changes the size of its opening depending on engine heat conditions. If the cooling system performance is degraded, engine performance may be compromised. For example, if the thermostat does not open once the coolant reaches a certain temperature, engine may overheat. Also, if the thermostat is stuck open, the engine will not heat up properly, a rich fuel-air mixture may be supplied longer than necessary, thus potentially degrading emissions and fuel efficiency. Similarly, if the engine coolant temperature sensor is not indicating actual coolant temperature, emissions, fuel efficiency and driver satisfaction will be degraded. One method of diagnosing the engine coolant system is described in U.S. Pat. No. 4,274,381. Engine coolant temperature is inferred from a temperature sensor such as the temperature sensor of the catalytic converter. This inferred value is compared to the value read by the coolant temperature sensor. If the two values are not the same, sensor degradation is indicated. Then, a signal corresponding to the output of the engine coolant temperature sensor under normal engine operating conditions replaces the output of the degraded coolant temperature sensor.
The inventor herein has recognized a disadvantage with this approach. In particular, this method does not diagnose the cooling system thermostat. As long as the readings from the two sensors agree, the system is assumed to be functioning properly. However, if engine operating conditions indicate a certain coolant temperature level, and the actual coolant temperature is different, the thermostat may be stuck in an open or closed state. If the thermostat performance is degraded, efficient temperature levels will not be maintained under all operating conditions, and thus, vehicle performance, fuel efficiency and emission control may be degraded. Also, the prior art does not use a cooling system model to estimate what coolant temperature should be based on engine operating conditions. Further, it does not take into account changes in the cooling system based on the open or closed state of the thermostat.
An object of the present invention is to provide a method for diagnosing a cooling system in an internal combustion engine, and in particular to diagnosing the engine thermostat.
The above object is achieved and disadvantages of prior approaches overcome by a method for diagnosing a thermostat in an internal combustion engine, the method comprising: estimating an engine coolant temperature based on an operating condition and a characteristic of the thermostat; reading said engine coolant temperature; and determining that the thermostat performance is degraded if said reading agrees with said estimate in a first operating region, and said reading disagrees with said estimate in a second operating region.
An advantage of the above object of the invention is that a method of diagnosing the engine thermostat is developed. By determining agreement in one region and disagreement in a second region, it is possible to isolate thermostat degradation from engine coolant temperature sensor degradation.